Preparation of potassium ferricyanide



Feb. u, 1947. R- GRAVELL l2,415,792

PREPRATIQN OF POTASSIVUM FERRICYANIl-)E Filed sept. 19, 1945 fxrP/rcr/OA/ rissa' Y l 510i mw che Wm oef/v4 MPM@ Nap l? (C095 sa 07m/v1 soz/0 KCI Patented Feb. 11, 1947.

PREPARATION OF POTASSIUM FERRICYANIDE Rogers Gravell, Linden, N. J., assigner to American Cyanamid Company, New York, N. Y., a corporation of Maine trice Application September 19, 1945, Serial No. 617,262

2 Claims. l

The present invention relates to the preparation of alkali metal ferri and ferrocyanides and specifically to K2Na2Fe(CN)s, lieNaFeNM and KsleN) s or any of them.

The principal object of the invention is the preparation of one or more of the above substances from a cheap source of raw materials as a complete cyclic process without undue waste and without relying upon any other or related process to take values which would otherwise be lost.

The alkali metal ferri and ferrocyanides are of particular value in the industry in that they are used extensively as photographic reagents and as intermediates in the production of iron blue complexes for pigments or the like.

These complex iron cyani'des may be readily prepared from a cyanogen compound such as crude calcium cyanide and ierrous sulfate but due to the chemical ainity of the Fe(Cl\T) e radical for the calcium, diiculty has heretofore been encountered. in forming the alkali metal salt. It has been discovered that by judicious choice of the intermediate alkali or alkaline earth metal ferrocyanide, paying particular attention to its ease of formation and insolubility in the menstruum in which it is formed, a perfect and operative process results.

For instance, calcium ferrocyanide may readily be formed by reacting a solution of calcium cyanide with ferrous sulfate. One of the calcium atoms only may be readily replaced by potassium through reaction with potassium chloride, for instance. The other calcium atom cannot be replaced as such with potassium even though large quantities of potassium chloride are added at this stage. On the contrary, however, this remaining calcium atom may be readily replaced with sodium as by reaction with soda ash details as hereinafter more fully described and shown in the drawing in which the single ligure is a flow sheet showing the preferred method of carrying out the invention.

Referring now to the flow sheet, crude calcium Considerable heat is developed as the reaction progresses so that only a small quantity of steam .is required to bring the temperature of the eX- traction menstruum to about 200 F. which is desired in order to bring about a comple@c and adequate extraction. The resulting slurry is filtered on a continuous ilter of the Oliver type or the like and the calcium sulfate sludge washed with water at about 126 F. The wash water from the cake 'may be returned to the extraction vessel for an additional cycle. Where desired and in order to improve the speed of iiltration, a small amount of calcium carbonate sludge, obtained later in the process, may be added to the slurry before filtration although this is not necessary.

This calcium ferrocyanide liquor contains some undesirable sulfate, usually about 0.3% calculated as potassium sulfate. Since the sulfate content of such liquors should be reduced to a minimum to produce good nal products, the liquors may be passed to a treating tank, sul"- cient barium chloride added to precipitate the sulfate as barium sulfate, and the sludge removed as by filtering.

to form K2Na2Fe CN a rThis material may "10 The thus puriiied calcium ferrocyanide is then either be recovered as such in solid form or one transferred to a precipitation tank, heated to of its sodium atoms may be then knocked out boiling with a steam coil and treated with an with chlorine to form K2NaFe CN e or this latter excess of solid potassium chloride with agitamateria-l may have its single sodium atom retion. The calcium ferrocyanide liquor added to placed With DOELSSUYH through reaction with 4" the precipitation tank contains about lS% of potassium chloride to form KsFe(CN)6. While calcium errocyanide and from 6 to 7% sodium at rst blush this would appear to be a roun'dchloride, the latter having been present as an about method of arriving at an end product, yet impurity in the crude calcium cyanide. by so doing advantage is taken of the ability r At this stage, there may also be added the of one material to replace another in a ferro 00 mother liquor containing substantial quantities or ferricyanide while at the same time choosing of K3Fe CN 5 from a later part of the process a material to be formed at that stage which .is and for the purpose of recovering Values therein. more insoluble in the menstruum than the by- Under the circumstances, approximately 20% of products or impurities present there. the ferrocyanide present at this stage 'may con- The invention further contemplates additional 55 sist of ferrccyanide which has thus been recycled.

The quantity of KCl added represents an excess of about 30% above the theoretical required for the calcium ferrocyanide liquor used, based on the following reaction:

The calcium ferrocyanide liquor contains reducing substances which ordinarily will reduce most of the ierricyanide to the ferro state. Any ierricyanide not so reduced may be converted by adding amounts oi sodium hydrosulde solution as required. The bulk of the ferrocyanide is precipitated as the sparingly soluble calcium potassium ferrocyanide and may be recovered by ltration, the cake being washed with a small amount of Water. The Washings and the liltrate containing the calcium chloride, the excess potassium chloride and the sodium chloride derived from the crude cyanide and recycled mother liquor, are discarded.

The cake of calcium potassium ferrocyanide is then transferred to a. heated mixer equipped with an agitator and slurriecl with weak liquor obtained from a previous mixer batch. Soda ash solution in slight excess as determined by test, is added according to the following reaction:

and the Whole mass heated to boiling by means of a steam coil or the like. is iiltered to recover the precipitated calcium carbonate which is Washed with Water sufiicient to yield the volume of Weak liquor necessary for preparing the next batch. About of the ferrocyanide present in any batch at this stage consists of liquor recirculated in this way.

The mother liquor from the iiltrate contains K2Na2Fc(CN)c, having a content pf about 31% calculated as KBFMCN) c and may be used as such in the arts, or evaporated and the crystals recovered for use.

In the event that it is not desired to recover KzNazFeiCN) e, this mother liquor may then be transferred to a chlorinating tank, cooled to about 100 F. and oxidized by passing chlorine thereinto at such a rate that the temperature does not exceed 120 F. In the chlorinating tank, the following reaction occurs:

The end point is indicated by the virtual disappearance of ierrocyanide. The KzNaFeiCNlc liquor thus produced contains the equivalent of about 31% potassium ierricyanide plus about 8% ci sodium chloride derived from the ferrocyanide and excess soda ash. It may be ltered to remove a small amount or" insoluble matter formed in the previous steps and transferred to a glass lined vacuum evaporator. The solution is heated to about 150 F., the vacuum applied and the evaporation continued by the introduction of steam to the jacket at such a rate that the temperature remains at about 160 F. and does not exceed 165 F. Theseconditions may be readily achieved by maintaining the vacuum at from about 27 to 29 inches mercury. Evaporation is continued until about 41% of the total weight oi solution has been removed as Water, the amount of water being evaporated according to the exact original concentration so that the mother liquor obtained by cooling the slurry to 60 F. Will separate crystals of K2NaFe(CN) s. Where necessary, the evaporated liquor and its crystals may be transferred to a magma tank and chlorine added The resulting slurry to reoXidiZe any ferricyanide which may have been reduced to ferrocyanide in the evaporation process. The mother liquor contains about 12% of total chloride calculated as chlorine. The evaporated liquor, containing any crystals which have separated during the evaporation, is then cooled under vacuum to about 60 F. and the resulting slurry filtered so as to collect the crude crystals of K2NaFe(CN)6. They may be further puried by recrystallization from Water if desired. The crude crystals contain only about 4% water and small amounts of chlorides and ferrocyanides as impurities. When dried, the product contains 102% to 103% of ferricyanide calculated as potassium ferricyanide equivalent to 97 to 98% calculated as the double salt, with 0.3% to 0.6% chloride calculated as chlorine, and 0.3% to 0.6% ferrocyanide calculated as if: Fe(CN)e. The liquor from the formation of crude and purified crystals of K2NaFe(CN)6 may be recycled, as previously mentioned, to the solution of `calcium ferrocyanide for recovery of the values therein.

Where it is not desired to produce crystals from the K2NaFe(CN)s liquor but on the contrary to convert it to K3Fe(CN) c, the crude K2NaFe(CN) s liquor containing about 31% equivalent of K3Fe(CN) may be treated with 0.5 to 0.6 lb. of solid KCl to each pound of equivalent K3Fe(CN) 6 present according to the following reaction:

5. KaNaFe (CN) s+KCl=K3Fe (CN) s-l-NaCl It may then be evaporated until about 6% of the weight has been removed as water. The resulting solution, including any crystals which have separated during the evaporation, is then cooled to about 60 F, and the crystals of KzFeN) 6 separated as by ltration. The product consists essentially of IQFMCNM with about 4% of water and small amounts of chlorides and ferrocyanides as impurities. Actually it contains from 100 to 101% of ferricyanide calculated as K3Fe(CN)o, with 0.3 to 0.6% chloride calculated as chlorine, and 0.3 to 0.6% ferrocyanide calculated as lFe(CN)c-. Where desired, the crystals may be further purified by recrystallization from water.

From the above it will be apparent that a selfcontained process has been designed for the preparation of K2Na2Fe(CN)e, K2NaFe(CN) c and K3Fe(CN)s or any of them, recycling values into the process. As a result, improved yields are obtained with consequent reduction in costs over that heretofore thought possible.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not to be limited thereto but is to be construced broadly and restricted solely by the scope of the appended claims.

I claim:

1. In a method of making K3Fe(CN)s by eX- tra-cting crude Ca(CN)2 with water, adding ferrous sulfate thereto to form Ca2Fe(CN)6, adding solid KCl to the latter to form crystals of CaK2Fe(CN)s, separating out the latter, reacting the same with soda ash solution to form KgNazFeN) s and chlorinating the latter to form KzNaFeN) c, the improvement which includes adding solid KCl to the thus formed solution of KzNaFe (CN) s to form K3Fe(CN) c and recovering the latter through evaporation and cooling.

2. Thel method of claim 1 in which the mother liquor from the K3Fe(CN)6 recovery step is returned to cycle by addition to incoming Ca2Fe CN s liquor.

ROGERS GRAVELL. 

